(1) Field of the Invention
The invention relates to a method of fabricating semiconductor structures, and more particularly, to the formation of polysilicon resistors shielded from hydrogen intrusion.
(2) Description of the Prior Art
Integrated circuits are a combination of various active components, such as transistors, with passive components, such as resistors. Resistors used in integrated circuits are often formed from undoped or lightly doped polysilicon. Such polysilicon has a relatively high resistivity from which large value resistors can be constructed using polysilicon traces while taking minimal area.
Unfortunately, polysilicon resistors also exhibit a tendency to absorb hydrogen atoms to which they are exposed during semiconductor processing. Hydrogen is used as an atmosphere during annealing operations. This hydrogen is absorbed into the polysilicon at trap locations in the grain boundaries. The presence of the hydrogen atoms effectively inactivates the trap locations. This causes the resistivity of the polysilicon to decrease. In addition, because the exposure to hydrogen can vary depending on many parameters, changes in the actual resistance values of the various resistors on the integrated circuit are not consistent.
A cross-sectional view of a partially completed prior art polysilicon resistor is shown in FIG. 1. The schematic shows a semiconductor substrate 11. Field oxide isolation layers 12 are grown on and in the substrate 11. A lightly doped polysilicon resistor 13 overlies the field oxide 12. An interlevel dielectric 14 overlies the polysilicon resistor 13 and field oxide 12. A metal layer 15 contacts the polysilicon resistor 13 and partially overlies the interlevel dielectric 14. A plasma nitride layer 16, so-called because it is formed by plasma-enhanced chemical vapor deposition, overlies the interlevel dielectric 14 and metal layer 15.
During a high temperature annealing process, hydrogen ions (H+) 17 contained in the nitride layer 16 can diffuse through the interlevel dielectric 14 into the polysilicon resistor 13. Once in the polysilicon, these hydrogen atoms are absorbed at the trap sites and the alteration in the resistor value occurs.
Several prior art approaches attempt to prevent hydrogen atoms from diffusing into polysilicon resistors. In Silicon Processing for the VLSI ERA, Vol. 1, by Wolf and Tauber, p. 222, it is revealed that silicon nitride can serve to block the diffusion of atomic hydrogen. U.S. Pat. No. 5,530,418 to Hsu et al shows polysilicon and metal layers preventing hydrogen intrusion to a polysilicon resistor therebetween. U.S. Pat. No. 5,236,857 to Eklund et al discloses a silicon nitride insulating layer over a polysilicon resistor. U.S. Pat. No. 5,470,764 to Ikegami et al discloses a silicon nitride layer under a polysilicon resistor. The silicon nitride is formed by implanting nitrogen into the polysilicon and then annealing. U.S. Pat. No. 5,290,727 to Jain et al teaches a silicon rich silicon dioxide layer over a polysilicon resistor. U.S. Pat. No. 5,728,615 to Cheng et al discloses an anneal to make the atomic hydrogen diffusion into the polysilicon resistor more uniform. U.S. Pat. No. 5,759,887 to Ito et al teaches the use of a silicon nitride layer over the polysilicon resistor. Finally, co-pending U.S. patent application Ser. No. 09/234,096 (TSMC-98-169) to Hsu et al filed on Jan. 19, 1999 discloses the use of a low-pressure silicon nitride cap as a barrier to reduce resistance variations from hydrogen intrusion of high-value polysilicon resistors. This approach requires the deposition of an additional silicon nitride film over the surface of the wafer that can slow manufacturing throughput.